The purpose of this project is to clarify the role of afferent sensations (kinesthesis) and central perceptions of motor commands (the sense of effort) in the control of hand grasp. Afferent information is thought to play an essential role in some tasks (learning new movements, correcting for disturbances, making precise movements), but seems less necessary in others (simple movements, learned or repetitive movements). The relative contributions of afferent- and efferent-based strategies have not been quantified, however, and this project is designed to provide that information within the context of basic grasping tasks. The specific aims of the project are formulated in terms of an "equilibrium point" model of sensorimotor control. The model has been used in the past primarily to explain movement and posture, but it is also the only model of motor control that has been formulated explicitly to accommodate kinesthesis. The sensory behavior predicted by the model has not been tested directly, however. The key to testing the model is to directly compare the results of the proposed behavioral experiments (matching and magnitude estimation) with the mechanical properties of the hand (specifically, the compliance of the finger and thumb). The paradigm used here will require subjects to make bilateral force, effort or span matches of thumb-finger squeezes against equal and unequal loads. Subjects will make magnitude estimates at the same time, and the thumb-finger compliance will be measured by making small changes in the loading force and measuring the resulting change in finger span. The muscle compliance will also be derived from the matching data under the assumptions of the model. The derived compliance and the actual compliance will agree only if the subjects actually match effort. Therefore, there are three specific hypotheses to test. (l) When subjects match effort, the actual muscle compliance will be the same as the compliance derived from the force and position errors under the assumptions of effort matching and the equilibrium point model. (2) When subjects match force based on afferent information, the efferent commands will be mismatched in order to equate forces. The force errors will be small, and the compliance derived from them will be significantly higher than the actual compliance. (3) When subjects match position or span based on afferent information, the efferent commands will be mismatched in order to equate positions. The position errors will be small, and the compliance derived from them will be significantly lower than the actual compliance. The success or failure of these hypotheses will differentiate between kinesthetic- or command-based control for a particular task. Identification of the dominant strategy will make it possible, in turn, to reliably interpret the magnitude estimation results and help resolve discrepancies reported in the literature. The results of this investigation will help clarify the role of kinesthetic information in the sensorimotor function of the hand. Moreover, the information will assist in the rehabilitation of sensory function in the tetraplegia hand by providing guidelines for improving methods of device control (efferent command) or cognitive sensory feedback (kinesthesis) in a hand- ras neuroprosthesis.